[Amps] Modern Op-Amp shunt stabilizator power supply (for tetrodescreen).

[flynth at gmail.com]

Thank you Jim. Also Roger - W7TZ. You both show an arrangement I haven't
seen before. Most likely I'll use it.





On Mon, 28 Feb 2022, 21:37 Jim Garland, <4cx250b at miamioh.edu> wrote:

> Attached is the screen voltage regulator I designed for my 160m/80m
> duo-band amplifier using three GU74B tetrodes. From your msg, I suspect
> it’s similar in some ways to the circuit you’ve described. The circuitry is
> deceptively subtle, however, in that it provides protection against damage
> from grid-plate flashovers in the tubes, which can be a problem, especially
> with new (NOS) Russian tetrodes. I’ve used this circuit for more than five
> years and found it to be always stable, with tight regulation. The
> circuitry mounts on a small file card-sized PCB. The screen voltage is
> adjustable and I set it at a nominal 340V.
>
>
>
> If the attachment doesn’t come through (I’m away from my home QTH and have
> limited internet coverage, here’s a link to the complete schematic package
> for the amplifier. Pse email me if you can’t get any of the diagrams to
> load!
>
>
>
> http://www.w8zr.net/160amplifier/images/Amplifier%20Schematic%20Package.pdf
>
> 73,
>
> Jim W8ZR
>
>
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows
>
>
>
> *From: *flynth at gmail.com
> *Sent: *Monday, February 28, 2022 10:50 AM
> *To: *amps at contesting.com
> *Subject: *[Amps] Modern Op-Amp shunt stabilizator power supply (for
> tetrodescreen).
>
>
>
> Did anyone figure out how to use a modern Op-Amp (not 748) in a mosfet
>
> shunt stabilizator for a tetrode screen power supply?
>
>
>
> This is what G3SEC said in his original article:
>
> "This is a very high-gain feedback loop, so it requires stabilization over
>
> a wide range of frequencies. An ordinary internally compensated op-amp is
>
> not suitable – in fact it will oscillate. The simple trick, courtesy of
>
> G4JZQ, is to use an uncompensated op-amp such as the 748 with heavy
>
> external compensation from the network R4-C1. (If you’re not familiar with
>
> the 748, it’s simply the good old 741 without its built-in compensation
>
> capacitor.".
>
>
>
> The closest to what I'm looking for is a F1FRV design that uses LM 358 that
>
> drives a bipolar transistor that then drives a mosfet. It is described here
>
> http://f1frv.free.fr/main1a_Tetrode_Linear_Amp.html
>
>
>
> I'm not sure why is F1FRV not having the oscillation problem described by
>
> G3SEK. Is it because the bipolar transistor he uses between the mosfet and
>
> opamp's output presents more of a resistive load rather than capacitive? If
>
> this is the case perhaps I'll have to use an external bjt transistor same
>
> as F1FRV.
>
>
>
> I like the simplicity of the original G3SEK design, but sourcing 748 op
>
> amps is a pain. Also I have a drawer full of many different kinds of modern
>
> op amps.
>
>
>
> Perhaps one could achieve similar compensation as is done in 748 in an
>
> internally compensated device? Miller compensation perhaps? Is this doable?
>
>
>
> To clarify, I don't plan on replicating the original G3SEK tetrode board
>
> design. I have different supply voltages. I have a different Mosfet. I have
>
> to change component values anyway. But I would really like to use a single
>
> mosfet and a single Op-Amp.
>
>
>
> L.
>
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